ER,
I wish I could give you better than approximations, but a landing area of about 3/4 - 1 mile would be typical.
I re-read Gordon Mackenzies book "Bird of the Islands", in which he flies a Short Sandringham (with P&W twin Wasps) around the South Pacific.
No actual distances are given, but there are references to "1/2 mile remaining after touchdown", "3 boats with lights @ 1/4 mile spacing marking the landing path".
HTH,
SD